Alkali chlorides and sulfates are well-known species induced fouling, slagging and corrosion on the heat transfer surface in biomass combustion/gasification process. This paper aims to study the condensation behavior of alkali compound vapors (ACVs) during wheat straw combustion in laboratory, without the participation of ash particles. The effects of surface temperature of Fe-Cr-Ni alloy probe on condensation morphology, thermodynamic characteristics of mixed salt vapors, and mineralogical variation of straw ashes are also discussed. The K, Cl and S mainly existed as KCl, K2SO4 and K2Ca(CO3)(2) in wheat straw ashes produced at 400 degrees C. The main released ACVs were potassium chlorides and sulfates. The form of condensation for mixed chloride vapors at surface temperatures of 400/520/600 degrees C is varied at 815 degrees C. These mixed salt vapors could condense either separately as cubic crystal particles by homogeneous nucleation, or together as one or two solid solutions by heterogeneous nucleation. The side length of KCl crystal particle condensed homogeneously was less than 2 mu m. The aggregation and growth among crystal particles at surface temperature of 400 degrees C were observable. The chlorides are released completely from wheat straw ashes at 815 degrees C, and carbonates may disappear as either decomposition or sulfation with SO2/SO3. When the evaporation temperature was 1100 degrees C, K2SO4 and Na2SO4 were released and then condensed as one solid solution with hexagonal crystal structure on the probe surface of 520 degrees C. Finally, we propose a condensing mechanism for ACVs depending on surface temperature. (C) 2016 Elsevier Ltd. All rights reserved.